The present invention is directed to a method for controlling a phase inversion in a liquid-liquid dispersion. In particular, the present invention is directed to the control of a phase inversion in a liquid-liquid dispersion through the application of electric fields. The present invention is also directed to the enhancement of mass transfer between two liquids in a liquid-liquid dispersion.
Liquid-liquid dispersions are made of two immiscible or partially miscible liquids and are used in many chemical processes. Liquid-liquid dispersions exist in many different areas. For example, in the oil and gas industry, there are oil and water emulsions, which usually have to be separated. Additionally, in the pharmaceutical industry, liquid-liquid dispersions are used for several purposes, including the transfer of a solute from one liquid to another and for polymerization reactions. Finally, liquid-liquid dispersions may be used in chemical separations and clean-up operations to remove a toxin or other undesirable compound from a liquid.
In a liquid-liquid dispersion, there are two liquid immiscible phases, one dispersed as droplets and the other continuous. There are several chemical processes, including, for example, chemical separations and polymerization reactions, in which it is desired to have a high volume fraction of each phase. As one increases the dispersed-phase volume fraction, however, in a liquid-liquid system, there is a risk of phase inversion.
Phase inversion in liquid-liquid dispersions is the phenomenon in which the dispersed phase becomes continuous and the continuous phase becomes dispersed. It is often referred to as a xe2x80x9ccatastrophicxe2x80x9d phenomenon because (i) when phase inversion occurs, the physical properties of the dispersed and continuous phases change abruptly and (ii) inversion back to the initial state of the dispersion involves interruption of the process.
Phase inversion is affected by a variety of factors including the physical properties of both liquid phases in the dispersion, the geometry and materials of the equipment, and the initial and operational conditions. In an aqueous/organic dispersion, if the volume fraction of the organic phase is plotted versus agitation intensity, a region termed the xe2x80x9cambivalence regionxe2x80x9d exists in which either phase can be continuous or dispersed. The initial conditions usually determine which phase (aqueous or organic) will be continuous or dispersed. For example, it has been reported in the literature (Gilchrist et al., 1989) that the liquid phase which is continuous is likely to remain the continuous phase once agitation is initiated. If the volume fraction of the organic phase is greater than the upper bound of the ambivalence region, then the organic phase is continuous and the aqueous is dispersed. Similarly, if the volume fraction of the organic phase is below the lower bound of the ambivalence region, the aqueous phase is continuous and the organic is dispersed.
Effects of physical properties such as density, viscosity, and interfacial surface tension on phase inversion and the ambivalence region have been examined by several investigators (Quinn and Sigloh, 1963; Selker and Sleicher, 1965; Luhning and Sawistowski, 1971; Guilinger et al., 1988; Norato et al., 1998). Effects of equipment geometry and materials of construction have also been examined (Gilchrist et al., 1989; Kumar et al., 1991). A phase-inversion delay time has been investigated by Gilchrist et al. (1989), Kato et al. (1991), and Pacek et al. (1994).
However, these studies have failed to discover a method by which these phase inversions may be stopped, or reversed quickly if the phase inversion has started. As mentioned, phase inversions often are highly undesirable. Typically, when a phase inversion occurs during a process, the process must be shut down, thereby resulting in time and expense for correcting the phase inversion and for the loss in operating time.
Additionally, the prior art has failed to discover methods for controlling phase inversions. These methods would enhance the use of liquid-liquid dispersions as previously discussed by permitting higher volume fractions of the dispersed organic phase without inducing phase inversion. Also, these methods could enhance mass transfer rates from one phase to another to improve the effectiveness of using liquid-liquid dispersions in polymerization reactions and chemical separations.
Some of the prior art has examined the use of applied electric fields in liquid-liquid dispersions. Electric fields have been used to enhance coalescence and removal of small aqueous droplets from an organic continuous phase (e.g., Ptasinski and Kerkhof, 1992). The enhancement of drop coalescence is due to polarization of the conductive droplets in the electric field and mutual attraction (Baygent, et al., 1998). However, while these methods have shown to be effective at separating one liquid from another, they do not teach or suggest a method by which phase inversions may be prevented or reversed.
Accordingly, what is needed is a method of controlling phase inversions in liquid-liquid dispersions to prevent an inversion from occurring or reverse the inversion should one occur. Also what is needed is a method for permitting the increase in the volume fraction of a dispersed organic phase without causing a phase inversion. Finally, what is needed is a method to enhance mass-transfer of a solute from one phase to another in a liquid-liquid dispersion.
The present invention seeks to provide a method of controlling phase inversions in liquid-liquid dispersions to prevent an inversion from occurring. The present invention also seeks to provide a method of correcting a phase inversion without shutting down the ongoing process. Additionally, the present invention also seeks to provide a method of increasing the volume fraction of a dispersed organic phase without causing a phase inversion. Finally, the present invention seeks to provide a method for enhancing mass transfer of a solute from one phase to another in a liquid-liquid dispersion.
In accordance with the present invention, these objects are accomplished by a method for controlling phase inversions in a liquid-liquid dispersion. By being able to control the dispersion, it is possible to prevent a phase inversion from occurring or reverse one that has already occurred. Additionally, by being able to prevent a phase inversion, it is possible to achieve a higher volume of the dispersed phase without causing phase inversion. Finally, by causing and reversing phase inversions in some chemical processes, it is possible to enhance mass transfer of a solute from one phase to another.
The present invention controls phase inversions through the application of an electric field to the liquid-liquid dispersion. The liquid-liquid dispersion will typically comprise a continuous aqueous phase and a dispersed organic phase. The electric field enhances the coalescence efficiency and the coalescence rate of the aqueous phase, thereby causing a continuous aqueous phase to remain continuous, even at higher volume fractions of the dispersed organic phase. Additionally, if a phase inversion has occurred, the electric field will enhance the coalescence of the dispersed aqueous droplets to cause the aqueous phase to become continuous, thereby reversing the phase inversion. Finally, by alternately applying and disengaging the electric field, it is possible to alternately cause and reverse phase inversions within a liquid-liquid dispersion, thereby enhancing mass transfer rates of a solute from one phase to another.
The electric field is applied to the dispersion with the objective of maintaining a dispersion in which the more conductive phase, typically an aqueous phase, is continuous and the less conductive phase, typically organic, is dispersed. This objective is achieved using two electrodes which are introduced into the dispersion. The electrodes are connected to a power supply. An electrical signal applied to the electrodes results in an aqueous-continuous/organic-dispersed dispersion. If initially the dispersion is aqueous-dispersed/organic-continuous, the electric field enhances coalescence of the aqueous droplets, which eventually causes phase inversion. On the other hand, if the dispersion is aqueous-continuous/organic-dispersed, the electric field ensures that the phase inversion will not occur.
If the composition of the dispersion is located above the ambivalence region for the dispersion, then alternately applying and disengaging an electric field will result in alternately causing and reversing a phase inversion within the liquid-liquid dispersion. By turning the electric field on and off, the present invention may be used to better control and optimize processes that involve liquid-liquid dispersions.
These and other objects, features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended claims.